Monday 8 April 2019

Seven years of solar power: How valuable is our solar energy ?

I've been interested in solar energy since before I can remember. My first practical experience beyond things like solar powered calculators was in the mid 1980s when I put a square meter of surplus panels on the roof of my parent's home. These provided 12 V DC which I used, amongst other things, to charge the battery in my then quite new laptop computer. It wasn't until seven years ago, though, that we installed a large system on our home and the seventh anniversary of that system passed on April 5th.

Seven year summary
We have 16 panels each rated for 235 W output for a total of a 3760 W peak. In this part of the world it is usual to calculate the expected total output as the equivalent of 925 hours of full sun each year on the panels, giving an predicted output of 3478 kWh per year. In practice, over the last seven years our system produced 23601 kWh in total or an average of 3372 kWh per year.

Output per month over seven years. Red bars show our electricity consumption, blue bars show the production from our panels and the yellow line shows where we are in total now relative to where we began. Our production in is higher than our consumption on average, though obviously that is not the case in the winter. The kink in the yellow line from July 2018 is the period during which our inverter was not working (I repaired it myself)
Several things contribute to the slightly lower output relative to the estimate: Our panels are mounted at the angle of the roof and not at whatever the ideal angle might be, they face South West rather than directly towards the South. But the most damaging thing so far as the average is concerned is that our inverter failed last year and so we had no output at all for a few weeks during the sunnier than average month of July. Calculating out average output for the six years before the inverter problem we come to 3410 kWh per year which is within 2% of the prediction.

Return on investment
The monetary value of the electricity which we've generated is about 20 cents per kWh for that which we use ourselves and about 7 kWh for the excess that we export to the grid. We have exported about 1200 kWh in total for a value of €84 and consumed about 22400 kWh ourselves for which we would otherwise have had to pay about €4500. In total, then, about €4600 has been returned from our original €8000 investment. If the same rate of return continues then the system will have taken 12 years to pay for itself.

Our new smartmeter. When the photo was taken
everything electrical in our home was powered
from the panels and 2 kW was being exported
to the grid.
If the inverter fails again and this time we have to replace it then that will of course increase the repayment time. This might happen. However the panels themselves are expected to last much longer than the repayment period.

A smart meter
Something else which changed last year was that our electricity meter was replaced in December.

The new meter doesn't give us any real advantage over the old mechanical meter which span backwards just as enthusiastically as forwards for nearly seven years. The electricity costs us the same amount either way around.

Of course it does offer an advantage to the electricity company because they no longer have to ask us to read the meter or visit to do so themselves.

What the new meter does give me, though, is that it counts differently and therefore I have a little more information for future calculations. While we pay the same amount of electricity in peak and off peak periods, the meter displays them separately, for both inbound and outbound electricity. So now we can see how much electricity we consume at night time and we can see how much leaves our home rather than being consumed here.

Solar electricity is more valuable than average because it is
generated at peak times of consumption (source)
Perhaps after a year of recording this information I'll have something which which I can create an interesting graph. Thus far I have just three and a half months of this extra data, all of it from winter and spring. There's not much contrast to see.

What has long been obvious of course, and is also visible even in the small amount of data which I have now, is that our electricity is generated only in the daytime and then mostly on sunnier days. As it happens, solar electricity remains quite valuable in the Netherlands because this daytime generation corresponds reasonably well with peak usage. That means that the electricity which we export, most of what we generate, is almost certainly of use to someone else.

The first thing I made the prototype
hardware do was wiggle a GPIO and
then send serial data as shown here.
Software for a smart meter
I'm late to have a smart meter installed but as it happens, I spent a few months two years ago working on software for the prototype smart meter hardware of one of several competing manufacturers. I'm almost certain that the meter that I have now is not related to the manufacturer for whom I did the work, so my code is almost certainly not involved in reporting my own electricity usage, but as I'll never open the box of the meter and look inside, I'll never know for certain.

It was quite an interesting project for a while because it was like a return to the old days for me, when I worked on 8 bit processors and embedded software. This work was with what for me was a new processor, the Renesas RL78. It's a quirky 8/16 bit design. The RL78 assembler is styled so that the source code looks a bit like that for the Z80, but the processor is really completely different. In total I had 32 kB of flash and 4 kB of RAM to work with. These days that doesn't sound like much at all but that means this device has about the same amount of memory as the guidance computers which took Apollo to the moon so it's enough to do great things. In this case it's in a finger nail sized package which consumes micro-watts, and the little RL78 can of course compute many times faster than the AGC.

Power for a home and a business
We generate more than enough electricity to power both our home and our business, though of course I always point out that we're not actually doing so at night, or on darker days in the winter. Anyway, if you want a support a solar powered bicycle business which uses no powered vehicles then you can do so here:

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